Because he’s committed to the Democratic agenda?
My change in party affiliation does not mean that I will be a party-line voter any more for the Democrats that I have been for the Republicans. Unlike Senator Jeffords’ switch which changed party control, I will not be an automatic 60th vote for cloture. For example, my position on Employees Free Choice (Card Check) will not change.
Or because he just read some internal poll numbers:
Specter as a Democrat would also fundamentally alter the 2010 calculus in Pennsylvania as he was expected to face a difficult primary challenge next year from former Rep. Pat Toomey.
Or, in his own words:
I am taking this action now because there are fewer than thirteen months to the 2010 Pennsylvania Primary and there is much to be done in preparation for that election.